Studies have continued on the mechanism by which specific pathogenic bacteria evade killing by the serum complement system. Experiments with serum resistant E. coli and salmonella demonstrated that the C3 which was deposited on the bacterial surface by the alternative complement pathway bound predominantly to lipopolysaccharide (LPS) molecules through an ester linkage. Furthermore, C3 bound almost exclusively to the subset of LPS molecules bearing the longest O-polysaccharide (O-PS) side chains. Growth of salmonella under conditions which rendered the organism sensitive to serum killing resulted in attachment of C3 to molecules of LPS bearing shorter O-PS side chains. We concluded that LPS sterically hinders access of complement to otherwise susceptible sites on the outer membrane. Studies with 3 isogenic strains of E. coli varying in outer membrane composition and serum sensitivity showed that killing by complement was associated with deposition of C9 on the outer membrane in a form resistant to elution with salt or trypsin. Experiments have also continued on the mechanism of resistance to serum killing in Neisseria gonorrhoeae (GC). We demonstrated that bactericidal but not non-bactericidal C5b-9 was associated with distinctive outer membrane proteins when extracted from serum treated GC with detergent. Additionally, bactericidal and non-bactericidal C5b-9 had different associations with outer membrane constituents by electron microscopy. Finally, we found that monoclonal antibodies directed against identical or closely related epitopes on gonococcal protein I varied markedly in bactericidal activity, despite leading to equivalent complement deposition on the bacterial surface.